According to the traditional, and until now the only feasible method within the cement industry of sintering pulverulent materials, the cement raw material is introduced in the form of a layer of material, and sintered on its way through a rotary kiln, possibly co-currently but usually countercurrently to a flow of combustion gas passed through the rotary kiln and generated by combustion of fuel introduced at an air inlet end of the rotary kiln. Usually, the raw materials are subjected to preheating and calcination, i.e., decomposition of CaCO.sub.3 to CaO and CO.sub.2, in suspension outside the rotary kiln. However, at least the heating from calcination temperature to sintering temperature as well as the sintering proper take place in the rotary kiln.
This type of rotary kiln, however, is not an ideal apparatus. The essential drawback is the relative inefficient heat transfer during the process of heating the material to the treating, e.g., the sintering temperature. Because of this the machinery dimensions are necessarily large, resulting in high initial apparatus costs, a substantial heat loss and a considerable thermal inertia leading to long starting up periods and control problems as well.
Another disadvantage of the rotary kiln is the rather limited number of variable process parameters available; as a result of which it is difficult at the same time to optimize the heating up process and the thermal treating process.
Numerous proposals for eliminating these drawbacks have been made and can be classified in two groups:
The first group comprises methods for obtaining an improved heat economy of the rotary kiln.
The second group comprises attempts to replace the rotary kiln by another more efficient heat treating apparatus.
The first group includes a method for improving the heat exchange between particles of raw material and the hot combustion gases by alternatively lifting and dropping the particles during their passage down the kiln by means of a trough-shaped conveyor flights to produce curtains of falling particles extending across the kiln as described in U.S. Pat. No. 3,799,735 to Jensen. An improved heat economy is thus achieved but it cannot be said that the above drawbacks of the traditional rotary kiln have been eliminated satisfactorily.
To the same group belong two methods in which the rotary kiln is also used as a suspension preheater, as described in Japanese Patent Publication No. 3916 to Mamoto of 1963 and British Patent Specification No. 1,396,402 to KHD.
The Japanese Patent publication relates to a process and kiln apparatus for burning cement wherein the heat transfer to the non-preheated material delivered to the kiln is sought to be maximized. According to this process cement raw materials are introduced either at the forward or at an intermediate position of the kiln suspended in a stream of hot air and allowed to be heated while flowing with the kiln gases towards the rear end of the kiln. Before the kiln gases reach the rear end of the kiln the material is supposed to be precipitated onto the kiln floor inside the kiln and the precipitated material is then subjected to further heat treatment while moving along the inclined kiln bottom towards its lower-lying forward end. For many reasons this method has not been in practical use. In order to obtain proper precipitation of the suspended material excessive kiln dimensions both with respect to kiln length and kiln diameter at the rear end would be necessary.
Another process and kiln apparatus, wherein the material passes through a reaction drum, e.g., a rotary kiln twice, first in suspension and then in a precipitated state has been suggested. According to British Patent Specification No. 1,396,402 the material is passed through a reaction chamber constructed as a rotational drum provided with a burner, i.e., a rotary kiln, as a suspension in a gas, withdrawn from the rotary kiln in suspension, precipitated from the gas in a separator, and reintroduced into the rotary kiln in the form of a layer of material and further heat treated while moving along the inclined kiln bottom.
Instead of arranging a burner in the rotational drum a combustion chamber may be provided upstream of the reaction chamber. In this case the material is suspended in and heated by the hot exit gas from the combustion chamber.
In both cases the precipitation problems inherent in the Japanese method are solved, but these methods are not suited for treating sticky materials such as cement raw meal at a temperature close to the sintering temperature because the material will build up in the precipitator and quickly block it.
The second group comprises proposals for abolishing the rotary kiln entirely.
Thus, U.S. Pat. No. 2,776,132 to Pyzel discloses a method of manufacturing cement clinker according to which cement raw meal and fuel are introduced into a fluid bed where the heat generated by combustion of the fuel evokes partly calcining of raw meal and partly heating of the calcined raw meal to the temperature (approximately 1400.degree. C.-1450.degree. C.) at which the material sinters into cement clinker.
A later issued U.S. Pat. No. 3,013,786 to Pyzel discloses a similar method differing from the previous one in that the raw material is calcined not in a fluid bed, but suspended in a hot gas, whereafter the calcined material is separated from the gas and sintered in a fluid bed under simultaneous addition of fuel.
By both methods an impoved heat economy is obtained because the heat transfer in suspension and in a fluid bed is far more efficient than in a rotary kiln. On the other hand another significant disadvantage is encountered which has prevented these methods from ever having been of practical use. The pulverulent materials are partly molten and extremely sticky at the sintering temperature. Consequently, a fluid bed with such material has a marked tendency to form cakings, leading to frequent interruption in operation.
Previously, e.g., in U.S. Pat. No. 2,489,211 to Witt it has been proposed to sinter cement raw meal in suspension by head on collision of a flow of raw meal suspended in air with a flow of hot gas. This collision takes place in a reaction chamber in which the solid material is separated from the gas and sinks while the gas leaves the reaction chamber at the top.
This method has not been in practical use primarily because the material is not allowed sufficient reaction time for the mineral formation occuring at the sintering temperature, and secondly, because, should any sintering occur, the material would deposit on the walls of the reaction chamber during the separation phase as a consequence of its sticky nature, ultimately blocking the chamber.
British Patent Specification No. 959,446 to Heidelberg proposes another method of manufacturing cement clinker by suspension sintering, according to which fine raw material in a reaction zone is introduced into an ascending hot gas stream having a temperature sufficient to cause calcining and sintering of the raw material, and a flow velocity sufficient to keep the raw material suspended in the gas stream until calcined and sintered. The calcined and sintered material is precipitated from the suspension and withdrawn from the reaction zone by briefly interrupting the introduction of raw material and hot gas.
This method has found no practical use either, partly because its batchwise nature makes it very difficult to achieve a homogenous product, and partly because this method also leads to serious problems with cakings formed where the sticky material collides with the walls in the reaction zone.
U.S. Pat. No. 3,603,586 to Ritzmann discloses a continuous process with heating to sintering temperature and sintering of material in suspended state. The specification further discloses an apparatus for such heat treatment of fine material, e.g., cement raw material, comprising a multicyclone material preheater and a multicyclone cooler, and a burning section comprising a tubular firing chamber in which a suspension of preheated material is burned, the firing chamber being connected to a separating chamber constructed as an ordinary cyclone in which the product is precipitated.
This apparatus has not found practical use either within the cement industry, partly because of a marked tendency to the formation of cakings in the separating cyclone, and partly because realistic gas velocities and apparatus dimensions only allow a very short period of treatment which is insufficient for obtaining the desired mineral formation.
Finally, British Patent Specification No. 457,957 to Saint Jacques discloses a furnace for the treatment of pulverulent material said to be particularly suited for use in the production of cement clinker. The raw material, suspended in an air flow, is introduced tangentially at the top of this furnace which has the form of a vertically oriented cyliner. Further down additional air is introduced along with fuel. During the combustion the raw material is heated to sintering temperature in a suspended state. As the only exit for the gas is an outlet in the top of the furnace, the gas with the suspended raw material particles will follow a spiral path downwardly inside the furnace, after which the gas changes direction and leaves the furnace through the gas outlet, whereas the material particles are separated and sink to a rotating hearth provided with one or more tangential burners arranged at the bottom of the furnace.
This apparatus has not been used within the cement industry. One of the reasons for this is a strong tendency to the formation of cakings on the kiln walls. I have invented a method and apparatus for thermally treating, in particular, sintering, pulverulent raw materials which avoids the disadvantages of the prior art.